In recent years, digitization even in a radio broadcast has been advanced, and HD (High Definition) radio has been proposed as a type of terrestrial digital radio broadcasting.
A great advantage of the HD radio is as follows: when IBOC (In Band On Channel) to add a digital broadcast signal to an analog radio wave of an existing FM broadcast is employed, an identical broadcast can be received by a conventional analog type radio receiver and also a digital broadcast can be received by an HD radio receiver adapted to digital reception. In this manner, when an AM broadcast is received digitally, sound quality on a par with an FM broadcast is obtained, and when an FM broadcast is received digitally, sound quality on a par with a CD is obtained.
Further, as a basic function of HD radio, a digitally received signal is output in an area where digital reception is possible, and a signal received in analog form is output automatically in an area where a digital signal cannot be received to prevent sound interruption or the like. More specifically, during a search, the radio receiver is tuned to a frequency at which an analog broadcast wave can be received, and an analog output wave is initially demodulated. Meanwhile, a determination is made as to whether or not a digital modulated wave (in other words, an IBOC broadcast wave) exists in the analog broadcast wave, and when a digital modulated wave exists, the IBOC broadcast wave is demodulated. Further, by way of a processing called “blending”, the demodulated analog broadcast wave is switched to the IBOC broadcast wave, and the IBOC broadcast wave is output. When no digital modulated wave exists, on the other hand, the demodulated analog broadcast wave is output as is.
In a conventional radio receiver, an electric field intensity of an IF signal is detected by an electric field intensity detector, a frequency of the IF signal is detected by an IF counter, and a search for an analog broadcast signal is performed by determining whether or not a desired analog broadcast signal is being received on the basis of the detected electric field intensity and frequency.